For comment/discussion...
Other things equal, the field values in the vertical plane radiation pattern
first "launched" by a monopole are a function of earth conductivity. But
the shape of the radiation pattern close to the monopole remains essentially
the same for all conductivities, as shown in the link below.
The fields in the NEC4 analysis below were calculated along a vertical
distance of 0-50 meters above the surface of the earth, at a horizontal
distance of 100 meters from the monopole. A vertical distance of 50 meters
for this chart is an elevation angle of 26.6 degrees from the monopole.
Note in the chart that the radiated fields at elevation angles
below 26.6
degrees are greater than the field
at 26.6 degrees. For the lowest
elevation angles, those fields are FAR greater than those shown in a NEC
"far-field" analysis, which for real earth go to zero field in the
horizontal plane.
At an elevation distance in the chart of 15 meters (8.5 degrees), the
difference in the field shown for 1 mS/m conductivity and that for sea water
conductivity is 2.6 dB -- probably less than intuition would expect.
It is the low-angle radiation directed toward the ionosphere from the fields
at ~this electrical distance from the monopole that can produce useful
skywaves having the greatest single-hop range.
http://i62.photobucket.com/albums/h85/rfry-100/Monopole_Surf_Wv_Compare.jpg 
